1. Field of the Invention
The present invention relates to an optical storage device for accessing an optical recording medium such as a phase change type of optical disk and an optical magnetic disk, and a liquid crystal device preferably applicable to such a optical storage device.
2. Description of the Related Art
An optical disk, such as a phase change type (PD) of optical disk and an optical magnetic disk (MO), is a portable recording medium having a high storage capacity, and is being watched as a recording medium of a personal computer. A possibility of higher density and larger capacity for such an optical disk is pursued.
To implement a larger capacity of an optical disk while a recording area of the optical disk is maintained as it is, there is a need to increase a recording density of the recording area or to establish a multi-layer construction of recording. In order to increase a recording density of the recording area, there is basically a need to reduce a condensing spot of a laser beam to be used, while there is considered means such as a magnetic resolution.
Generally, a diameter of a spot of a laser beam is in proportion to
xcex/NA (xcex:wavelength of light, NA: numerical aperture).
Therefore, to implement a higher density of recording for an optical disk, there is a need to use a laser (for example, a laser emitting light of blue) which is short in wavelength, or increase NA of an objective lens.
However, in the event that NA of an objective lens is increased, it involves a problem of a spherical aberration due to an unevenness in thickness of transparent protective layers on a surface of an optical disk when the optical disk is manufactured.
Particularly, since the optical disk is constructed as a storage medium which is detachably loaded, there is needed a transparent protective layer on a layer which is essentially necessary for a storage and a pick-up of information, such as a reflecting layer and a recording layer. The unevenness in thickness of the protective layer on the manufacture is of xc2x150 xcexcm or so as an unevenness on an individual optical disk (an unevenness as an individual difference) and is of xc2x110 xcexcm or so as a variation inside the same optical disk (an unevenness inside an individual). The unevenness in thickness of transparent protective layers on a surface of an optical disk causes a spherical aberration on light condensed on the recording layer, and this spherical aberration has a bad effect on recording and reading of a pit mark.
FIG. 1 is a diagram showing a spherical aberration RMS to an unevenness of a protective layer.
FIG. 1 shows results of calculations in case of NA=0.6 corresponding to the present DVD, and in case of NA=0.85. In a case where an unevenness in thickness of the protective layer on the manufacture is of +S50 m, if NA=0.6, it is within an aberration allowance. On the other hand, if NA=0.85, an aberration, which cannot be covered, occurs. Therefore, to implement a high NA of objective lens there is needed a mechanism for correcting the spherical aberration in accordance with an unevenness in thickness of the protective layer.
In order to satisfy such a requirement, there is proposed a scheme in which two objective lenses are used so that a spherical aberration is actively corrected by mechanically altering an interval between the two objective lenses (cf. Japanese Patent Laid Open Gazette Hei. 8-212579).
However, according to this proposal, a further mechanical driving for the objective lens is added. Thus, this is associated with a problem that a weight of a head portion of a pick-up is increased and a larger space is needed.
On the other hand, there is proposed a scheme in which a liquid crystal device is disposed in an optical path so as to correct an aberration (cf. for example, Japanese Patent Laid Open Gazette Hei. 8-212611, Japanese Patent Laid Open Gazette Hei. 9-128785).
As such a liquid crystal device, there are known two types of an electrode structure of two dimensional matrix configuration and an electrode structure patterned after a pattern associated with the aberration.
To implement a matrix configuration of electrode, there is a need to use a TFT. The TFT matrix-panel needs a very complicated manufacturing process, and thus this is associated with such a problem that it is obliged to increase greatly the cost.
On the other hand, with respect to the electrode structure patterned, it is associated with a problem that a phase distribution of light, which is formed by a distribution of index of refraction of the liquid crystal device, is fixed. Thus, there is a need to dispose the liquid crystal device as to an optical axis with great accuracy. This involves such a problem that a strict precision of an alignment is required. In order to correct a spherical aberration through a patterned electrode structure, an electrode structure having a concentric circle of pattern is adopted. However, to correct the spherical aberration with greater accuracy, if the concentric circle of pattern is given with greater definition, this causes a polarization of the baser beam to be disturbed. Thus, this is not suitable for correction of the spherical aberration when the objective lens of a high NA is adopted. Further, in the event that the concentric circle of pattern is given with greater definition, this causes the number of lead wires derived from a ring electrode inside the concentric circle to be increased, and thereby increasing a wiring area for the lead wires. This is associated with a problem on manufacture that the concentric circle of pattern cannot be formed per se.
Further, Japanese Patent Laid Open Gazette Hei. 9-128785 discloses the use of a strip shaped electrode. However, the use of a strip shaped electrode cannot almost correct the aberration from a view point of an aberration correction at the time of a high NA.
In the above description, the necessity for the aberration correction is explained in association with an unevenness in thickness of the protective layer of the optical disk. On the other hand, also when it is intended that an optical storage medium having a multi-layer construction of recording, that is, a plurality of information recording layers in a depth direction, is implemented, there is a need to actively correct the aberration due to the variation in depth.
While the above explanation is made in connection with the optical disk, the above-mentioned problems are applied to, for example, a tape-like shaped optical storage medium too, regardless of the disk configuration.
In view of the foregoing, it is an object of the present invention to provide an optical storage device capable of effectively correcting an aberration caused by a variation in a depth from a surface of an optical storage medium to a condensing point, even if the depth is varied, for example, in cases of an unevenness in thickness of a protective layer and the multi-layer recording, and a liquid crystal device capable of being preferably adopted to the optical storage device for a use of the aberration correction.
To achieve the above-mentioned objects, the present invention provides an optical storage device comprising:
a light source;
an irradiation optical system for leading light emitted from said light source to condense on a predetermined optical storage medium;
a photo detector for picking up a signal light carrying information stored in said optical storage medium to read the information, said signal light being condensed onto said optical storage medium and reflected on said optical storage medium;
a pick-up optical system for leading said signal light to said photo detector;
a liquid crystal device having first and second liquid crystal layers disposed in mid way of an optical path of said irradiation optical system and extending in parallel with a direction intersecting said optical path, a plurality of first electrodes for driving said first liquid crystal layer, said plurality of first electrodes extending a predetermined x-direction intersecting said optical path and arranged in a y-direction intersecting both said optical path and said x-direction, and a plurality of second electrodes for driving said second liquid crystal layer, said plurality of second electrodes extending the y-direction and arranged in the x-direction; and
a liquid crystal driver for applying controlled voltages to said plurality of first electrodes and said plurality of second electrodes of said liquid crystal device to correct an aberration of light to be condensed on said optical storage medium.
An important inventive feature of an optical storage device of the present invention resides in the point that the optical storage device adopts a liquid crystal device having a structure that two liquid crystal layers, each of which has stripe-like shaped electrodes arranged in a direction perpendicularly intersecting one another, are superposed. This feature makes drawing of lead wires from the electrodes easy and also makes it possible to facilitate a fabrication of the device. Further, according to the optical storage device of the present invention, it is possible to correct the aberration in accordance with an electric control and thereby permitting a very rough alignment. Furthermore, according to the optical storage device of the present invention, as will be described later in connection with the preferred embodiments, the optical storage device has a sufficient aberration correction ability.
In the optical storage device according to the present invention as mentioned above, it is acceptable that said irradiation optical system has an objective lens at a place adjacent to said optical storage medium, said objective lens comprising a plano-convex lens and an aspherical lens.
This feature makes it possible to easily implement a high NA of objective lens, and thus according to the optical storage device of the present invention, it is possible to give a sufficient aberration correction ability, even if a high NA of objective lens is used.
In the optical storage device according to the present invention as mentioned above, it is preferable that said liquid crystal driver applies voltages to said plurality of first electrodes and said plurality of second electrodes of said liquid crystal device, said voltages being controlled in such a manner that a phase distribution of light passing through said first liquid crystal layer in the y-direction is of a Kinoform structure and a phase distribution of light passing through said second liquid crystal layer in the x-direction is of a Kinoform structure.
This feature makes it possible to constitute the liquid crystal device with thin liquid crystal layers, and thereby contributing to higher operational speed of the liquid crystal device.
Further, in the optical storage device according to the present invention as mentioned above, it is preferable that said first and second liquid crystal layers of said liquid crystal driver are determined in their properties in such a manner that a normal of an altering surface of a liquid crystal molecular alignment in said first liquid crystal layer, due to a change in an electric field within said first liquid crystal layer according to changes of voltages applied to said first electrodes, and a normal of an altering surface of a liquid crystal molecular alignment in said second liquid crystal layer, due to a change in an electric field within said second liquid crystal layer according to changes of voltages applied to said second electrodes, are directed to a same direction.
Such a determination of properties of the first and second liquid crystal layers prevents a polarization state of the incident light from being changed by the liquid crystal itself.
Alternatively, in the optical storage device according to the present invention as mentioned above, it is preferable that said first and second liquid crystal layers of said liquid crystal driver are determined in their properties in such a manner that a normal of an altering surface of a liquid crystal molecular alignment in said first liquid crystal layer, due to a change in an electric field within said first liquid crystal layer according to changes of voltages applied to said first electrodes, and a normal of an altering surface of a liquid crystal molecular alignment in said second liquid crystal layer, due to a change in an electric field within said second liquid crystal layer according to changes of voltages applied to said second electrodes, establish a predetermined angle (e.g. 90xc2x0), and a wavelength plate (e.g. xcex/2 plate) for rotating a polarization direction of an incident light by the predetermined angle is disposed between the first liquid crystal layer and the second liquid crystal layer.
This arrangement also prevents a polarization state of the incident light from being changed by the liquid crystal itself.
Further, in the optical storage device according to the present invention as mentioned above, it is preferable that properties in alignment of liquid crystal molecules of said first and second liquid crystal layers of said liquid crystal device are of bend.
In a manufacturing process of a liquid crystal device, a property in alignment of liquid crystal molecules of liquid crystal layers of the liquid crystal device is selectable between a bend and a splay in accordance with a direction for a rubbing (mechanically) a substrate for supporting the liquid crystal layer. A selection of the bend contributes to a higher speed in change of alignment of liquid crystal molecules of the liquid crystal layer, that is, a higher operational speed of the liquid crystal device.
Furthermore, in the optical storage device according to the present invention as mentioned above, it is preferable that said irradiation optical system is an optical system which permits light beams emitted from said light source to pass through said liquid crystal device by one time while the light beams are condensed on said optical storage medium, and said first and second liquid crystal layers of said liquid crystal device are set up in their thickness such that phases of light emitted from said light source and passing through said first and second liquid crystal layers vary between 0 and 2xcfx80 under control of voltages applied to said first electrodes and said second electrodes, respectively.
A provision of a thickness of a liquid crystal layer, which permits a phase of a light to vary between 0 and 2xcfx80, makes it possible to correct an aberration adopting a phase change of the above-mentioned Kinoform structure, and also contributes to a higher speed in change of alignment of liquid crystal molecules of the liquid crystal layer.
Still further, in the optical storage device according to the present invention as mentioned above, it is preferable that said irradiation optical system is an optical system which permits light beams emitted from said light source to pass through said liquid crystal device on a reciprocation basis while the light beams are condensed on said optical storage medium, and said first and second liquid crystal layers of said liquid crystal device are set up in their thickness such that phases of light emitted from said light source and passing through said first and second liquid crystal layers by one time vary between 0 and xcfx80 under control of voltages applied to said first electrodes and said second electrodes, respectively.
In this case, the thickness of the liquid crystal layers becomes further half and thereby implementing a higher speed operation.
Still further, in the optical storage device according to the present invention as mentioned above, it is preferable that said liquid crystal device has a reflecting surface for reflecting a light incident onto said liquid crystal device and passing through both said first and second liquid crystal layers and for causing the light to pass through both said first and second liquid crystal layers again.
In the event that the liquid crystal device is used on a reciprocation basis, a provision of the above-mentioned reflecting surface on the liquid crystal device may avoid a necessity for preparing an additional reflecting mirror or the like. Thus, it is possible to contribute to miniaturization and low cost.
Still further, in the optical storage device according to the present invention as mentioned above, it is preferable that a width of each of said first electrodes of said liquid crystal device in connection with the y-direction has a size not less than a thickness of said first liquid crystal layer, and a width of each of said second electrodes of said liquid crystal device in connection with the x-direction has a size not less than a thickness of said second liquid crystal layer.
In the event that a width of an electrode is narrower than a thickness of a liquid crystal layer, an electric field, which is formed in the liquid crystal layer by a voltage applied to the electrode, extends within the liquid crystal layer. As a result, there is a possibility that an electric field distribution formed within the liquid crystal layer is greatly different from an ideal electric field distribution and thus it is difficult to expect a sufficient aberration correction. On the other hand, a formation of an electrode having a width larger than the thickness of the liquid crystal layer, as mentioned above, makes it possible to form a suitable electric field distribution within the liquid crystal layer.
Still further, in the optical storage device according to the present invention as mentioned above, it is acceptable that a part of said irradiation optical system is shared with a part of said pick-up optical system, said liquid crystal device is disposed at a portion for common use of said irradiation optical system and said pick-up optical system, light beams emitted from said light source are condensed via said liquid crystal device onto said optical storage medium, and the signal light carrying information stored in said optical storage medium, which is condensed onto said optical storage medium and reflected on said optical storage medium, is led via said liquid crystal device to said photo detector. Or alternatively it is acceptable that a part of said irradiation optical system is shared with a part of said pick-up optical system, said liquid crystal device is disposed at a portion other than a portion for common use of said irradiation optical system and said pick-up optical system, light beams emitted from said light source are condensed via said liquid crystal device onto said optical storage medium, and the signal light carrying information stored in said optical storage medium, which is condensed onto said optical storage medium and reflected on said optical storage medium, is led via an optical path, which is different from an optical path passing through said liquid crystal device, to said photo detector.
What needs an aberration correction is mainly an irradiation optical system side, either of the structures as mentioned above is acceptable.
Still further, in the optical storage device according to the present invention as mentioned above, it is preferable that said irradiation optical system has a beam splitter for splitting a light or determining a travelling direction of a light, and said liquid crystal device and said beam splitter are formed in a unitary body.
In the even that the irradiation optical system has a beam splitter, when the liquid crystal device and the beam splitter are formed in a unitary body, it is possible to reduce the number of parts and contribute to effective assembling and miniaturization of the device.
While the above explanation is concerned such a matter that the pick-up optical system of the optical storage device leads light reflected on the optical storage medium to the photo detector, it is acceptable to provide such an arrangement that a transparent type of optical storage medium is prepared, and the pick-up optical system of the optical storage device leads light passing through the optical storage medium to the photo detector
In this case, there is provided an optical storage device comprising:
a light source;
an irradiation optical system for leading light emitted from said light source to condense on a predetermined optical storage medium;
a photo detector for picking up a signal light carrying information stored in said optical storage medium to read the information, said signal light being condensed onto said optical storage medium and passing through said optical storage medium;
a pick-up optical system for leading said signal light to said photo detector;
a liquid crystal device having first and second liquid crystal layers disposed in mid way of an optical path of said irradiation optical system and extending in parallel with a direction intersecting said optical path, a plurality of first electrodes for driving said first liquid crystal layer, said plurality of first electrodes extending a predetermined x-direction intersecting said optical path and arranged in a y-direction intersecting both said optical path and said x-direction, and a plurality of second electrodes for driving said second liquid crystal layer, said plurality of second electrodes extending the y-direction and arranged in the x-direction; and
a liquid crystal driver for applying controlled voltages to said plurality of first electrodes and said plurality of second electrodes of said liquid crystal device to correct an aberration of light to be condensed on said optical storage medium.
In the optical storage device according to the present invention as mentioned above, it is acceptable that said optical storage medium has a plurality of information storage points in a depth direction, said liquid crystal driver applies voltages, which are controlled in accordance with condensing points in the depth direction of said optical storage medium, to the plurality of first electrodes and the plurality of second electrodes of said liquid crystal device, respectively, so that an aberration correction according to the condensing points in the depth direction of said optical storage medium is performed, said optical storage device further comprising:
a second liquid crystal device having third and fourth liquid crystal layers disposed in mid way of an optical path of said pick-up optical system and extending in parallel with a direction intersecting said optical path, a plurality of third electrodes for driving said third liquid crystal layer, said plurality of third electrodes extending a predetermined xxe2x80x2-direction intersecting said optical path and arranged in a yxe2x80x2-direction intersecting both said optical path and said xxe2x80x2-direction, and a plurality of fourth electrodes for driving said forth liquid crystal layer, said plurality of fourth electrodes extending the yxe2x80x2-direction and arranged in the xxe2x80x2-direction; and
a second liquid crystal driver for applying voltages, which are controlled in accordance with the condensing points in the depth direction of said optical storage medium, to said plurality of third electrodes and said plurality of fourth electrodes of said second liquid crystal device to perform an aberration correction according to the condensing points in the depth direction of said optical storage medium.
The present invention is also applicable to a multi-layer recording scheme of optical storage medium, that is, an optical storage medium having a plurality of information storage points in a depth direction.
Further, an optical storage device according to the present invention, it is possible to arrange it as an information writing dedicated-device to an optical storage medium.
In this case, there is provided an optical storage device comprising:
a light source;
an irradiation optical system for leading light emitted from said light source to condense on a predetermined optical storage medium;
a liquid crystal device having first and second liquid crystal layers disposed in mid way of an optical path of said irradiation optical system and extending in parallel with a direction intersecting said optical path, a plurality of first electrodes for driving said first liquid crystal layer, said plurality of first electrodes extending a predetermined x-direction intersecting said optical path and arranged in a y-direction intersecting both said optical path and said x-direction, and a plurality of second electrodes for driving said second liquid crystal layer, said plurality of second electrodes extending the y-direction and arranged in the x-direction; and
a liquid crystal driver for applying controlled voltages to said plurality of first electrodes and said plurality of second electrodes of said liquid crystal device to correct an aberration of light to be condensed on said optical storage medium.
In the optical storage device according to the present invention as mentioned above, it is acceptable that said optical storage medium has a plurality of information storage points in a depth direction, and
said liquid crystal driver applies voltages, which are controlled in accordance with condensing points in the depth direction of said optical storage medium, to the plurality of first electrodes and the plurality of second electrodes of said liquid crystal device, respectively, so that an aberration correction according to the condensing points in the depth direction of said optical storage medium is performed.
To achieve the above-mentioned objects, the present invention provides a liquid crystal device comprising:
first and second liquid crystal layers extending in a state that they are opposite to one another in parallel with a predetermined plane extending in an x-direction and a y-direction which intersect each other;
a plurality of first electrodes for driving said first liquid crystal layer, said plurality of first electrodes extending the x-direction and arranged in the y-direction; and
a plurality of second electrodes for driving said second liquid crystal layer, said plurality of second electrodes extending the y-direction and arranged in the x-direction.